1. Field of the Invention
The present invention relates to an encapsulated toner for heat-and-pressure fixing used for development of electrostatic latent images in electrophotography, electrostatic printing, or electrostatic recording.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer, subsequently exposing the layer to eliminate the charge on the exposed portion and visualizing the formed image by adhering colored charged fine powder, known as a toner, to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As indicated above, the toner must meet the requirements not only of the development process, but also of the transfer process and the fixing process.
Generally, a toner undergoes mechanical frictional forces due to shear force and impact force during the mechanical operation in a developer device, and deteriorates after copying from several thousands to several ten thousands of sheets. The deterioration of the toner can be prevented by using a tough resin having such a high molecular weight that it can withstand the above mechanical frictional forces. However, this kind of a resin generally has such a high softening point that the resulting toner cannot be sufficiently fixed by a non-contact method, such as oven fixing and radiant fixing by infrared rays, because of its poor thermal efficiency. Further, when the toner is fixed by a contact fixing method, such as a heat-and-pressure fixing method using a heat roller, which is excellent in thermal efficiency and therefore widely used, it becomes necessary to raise the temperature of the heat roller in order to achieve sufficient fixing of the toner, which brings about such disadvantages as deterioration of the fixing device, curling of the paper, and increase in consumed energy. Furthermore, the resin described above is poor in grindability, thereby remarkably lowering the production efficiency of the toner. Accordingly, the binder resin having too high of a degree of polymerization and also too high of a softening point cannot be used.
Meanwhile, according to the heat-and-pressure fixing method using a heat roller, the thermal efficiency is excellent, so that this method is widely used in various high-speed and low-speed copy machines. However, when the surface of a heat roller contacts the surface of the visible image, the toner is likely to cause a so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the surface of a heat roller is coated with a material having excellent release properties for the toner such as fluororesins, and further a releasing agent such as a silicone oil is applied thereon. However, the method of applying a releasing agent is likely to bring about various problems such as high costs and device troubles.
Although processes for improving the offset phenomenon by unsymmetrizing or crosslinking the resins have been disclosed in Japanese Patent Examined Publication No. 57-493 and Japanese Patent Laid-Open Nos. 50-44836 and 57-37353, the fixing temperature has not yet been improved by these processes.
Since the lowest fixing temperature of a toner is generally between the temperature of low-temperature offsetting of the toner and the temperature of the high-temperature offsetting thereof, the serviceable temperature range of the toner is from the lowest fixing temperature to the temperature for high-temperature offsetting. Accordingly, by lowering the lowest fixing temperature as much as possible and raising the temperature at which high-temperature offsetting occurs as much as possible, the serviceable fixing temperature can be lowered and the serviceable temperature range can be widened, which enables energy saving, high-speed fixing and prevention of curling of paper.
From the above reasons, the development of a toner having excellent fixing ability and offset resistance has always been desired.
Conventionally, various methods in which low-temperature fixing is achieved by using an encapsulated toner comprising a core material and a shell formed thereon so as to cover the surface of the core material have been proposed.
Among such toners, those having a core material made of a low-melting wax which is easily plastically deformable, as described in U.S. Pat. No. 3,269,626, Japanese Patent Examined Publication Nos. 46-15876 and 44-9880, and Japanese Patent Laid-Open Nos. 48-75032 and 48-75033, are poor in fixing strength, so that they can be used only in limited areas, although they can be fixed only by pressure. Further, in the case where toners having a liquid core material are used, when the shell strength is too low, the shell materials tend to break in the developer device and stain the inside thereof, though the toners are fixable with pressure alone. When the shell strength is too high, much pressure is necessary to break the capsule, and thereby the resulting formed images become too glossy. Thus, it has been difficult to control the strength of the shell materials.
Therefore, as a toner for heat-and-pressure fixing, an encapsulated toner for heat roller fixing has been proposed, which comprises a core material made of a resin having a low glass transition temperature which serves to improve the fixing strength, though blocking at a high temperature may take place if used alone, and a shell made of a high-melting point resin wall which is formed by interfacial polymerization for the purpose of imparting a blocking resistance to the toner. However, in Japanese Patent Laid-Open No. 61-56352, this toner cannot fully exhibit the performance of the core material, because the melting point of the shell material is too high and also the shell is too tough and not easily breakable. On the same line of the technical idea as that described above, encapsulated toners for heat roller fixing with an improved fixing strength of the core material have been proposed (see Japanese Patent Laid-Open Nos. 58-205162, 58-205163, 63-128357, 63-128358, 63-128359, 63-128360, 63-128361, and 63-128362). However, since these toners are prepared by a spray drying method, a higher load to the equipments for the production thereof becomes necessary. In addition, they cannot fully exhibit the performance of the core material, because they have not come up with a solution for the problems by the shell material.
Further, in the encapsulated toner proposed in Japanese Patent Laid-Open No. 63-281168, the shell is made of a thermotropic liquid crystal polyester, and in the encapsulated toner proposed in Japanese Patent Laid-Open No. 4-184358, a crystalline polyester is used. Since each of the polyesters used in these references is not amorphous, the resin sharply melts. However, the amount of energy required for fusion is large, and the Tg of the core material is also high, thereby making the fixing ability of the resulting toner poor.
As explained above, although encapsulated toners produced using various materials and production methods have been proposed, a toner satisfying all of the features such as low-temperature fixing ability, the offset resistance, the blocking resistance, and the stress resistance in a developer device has not yet been found. Particularly, no disclosure has yet been made to give quantitative evaluations of physical properties of the encapsulated toner which satisfy the above features.